The present invention relates to a mechanical device which can be used in particularly to achieve the synchronous movement of a seat with respect to the backrest of a chair.
As it is commonly known, the seat and the backrest of conventional chairs are provided on separate frames which are interconnected so that an inclination applied to the backrest is matched by a movement of the seat along an axis which is approximately parallel to the ground resting axis of the chair.
This solution is not ergonomic, since the user is subjected to pressure at the calves.
Moreover, the backrest of conventional chairs can oscillate with respect to the seat, which is fixed: even these solutions, however, are not ideal, since this inclination of the backrest can make the user slide at the seat, producing a condition which is ergonomically incorrect and possibly lifting the user's clothing at the backrest. U.S. Pat. No. 5,348,371 discloses a partial solution to this drawback, by providing a mechanical device for the synchronous movement of the seat and backrest of a chair, which substantially comprises a first support bracket pivoted, at one end, transversely to a corresponding end of a second fixing bracket for a seat, which is pivoted at the other end, transversely and eccentrically with respect to the axis of a central column which protrudes from a rotating base, to a third anchoring bracket for the backrest; said anchoring bracket has, at one end, slotted guides for its oscillation with respect to the first and second brackets and means for selectively locking the mutual position of the first, second and third brackets.
Although this solution is undoubtedly valid, it has drawbacks: the slotted shape of the end of the third bracket is a weak point of the structure, in that in order to comply with recent standards and the associated fatigue tests it is necessary to consider a thermal treatment of said ends in order to harden them and therefore ensure that they are not subject to deformations or breakage.
A further drawback that can be observed in this conventional embodiment is the fact that the movement of the backrest with respect to the chair is not always gradual and is sometimes sudden.
A further drawback of this prior art is the fact that at the first bracket there is an anchoring bush for the central column, a spring being arranged coaxially thereto and being accordingly interposed between the first bracket and the second bracket; this may causes the mutual movement of the first, second and third brackets not to be gradual.